The company has one system that manages inbound and another do outbound filtering.

The email gateway failed this week, it caused 3 hours delay.

Outbound did not work, it frozen. Only the inbound (company's domain) worked.Therefore, we are trying to find out if there is any multiple spam system that would be able to handle future outage in case that an email gateway fails again.

I'm a little unclear as to what you are asking, but do you want to take a look at SpamStopsHere Enterprise Edition? It includes multiple mailserver support and outbound filtering. It is hosted, so it's independent of your mailservers. And if your inbound server is down, we spool your incoming mail for up to 5 days and then deliver spooled mail when you're back up.

You can try it free for 30 days with a quick signup and MX switch. It includes 24/7 live support, even during your trial. Our support team would be happy to help out. You also get a nice Spiceworks discount for the first year of service with promo code SPICY15OFF.